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June 22, 2006

Amber-preserved Web Shows Early Spider Evolution

by Sam Savage

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - A tiny piece of spider web, frozen in a drip of tree sap 110 million years ago, shows spiders that wove perfect orb webs evolved much earlier than had been believed, scientists reported on Thursday.

A second study looking at spider genetics suggests that web-weaving arachnids evolved as long as 136 million years ago, in the Cretaceous period, when dinosaurs roamed and mammals were small, scurrying things.

Both studies, published in Friday's issue of the journal Science, shed light on the evolutionary forces that shaped the biology of insects and spiders that live today.

David Grimaldi, curator of invertebrate zoology at the American Museum of Natural History in New York, and colleagues in Spain found the bit of web, complete with prey, in a piece of amber in Spain.

"This is the oldest record of an actual web with prey items in it," Grimaldi said in a telephone interview.

The amber contains 26 web strands grasping a mite, a fly, a wasp leg and a beetle. Droplets of web glue are visible.

"It's a relatively small piece of amber. What is amazing is how much you can infer from the detail that has been preserved," Grimaldi said.

There is enough there to show that the web was an orb web -- the classic structure with concentric circles joined by radiating "spokes." Orb spiders are just one group among spiders. Others weave tangled "cobwebs" or jump on their prey.

"The advanced structure of this fossilized web, along with the type of prey that the web caught, indicates that spiders have been fishing insects from the air for a very long time," said Grimaldi.

"Spiders today have a huge impact as predators on insect populations, along with birds and bats."

A third team found one of the spiders that could have woven the web in another piece of amber in Spain. David Penney of the University of Manchester in Britain and Vicente Ortuno of the Universidad de Alcalo in Madrid date the amber to 115 million years ago.

The half inch (2 mm) long spider, which they name Mesozygiella dunlopi, looks very modern, they report in the journal Biology Letters.

Researchers had believed the orb web evolved twice but the two studies suggest it evolved early on and that later spiders lost the ability to make the geometrically precise framework.

"A lot of people had said over the years that the orb web was a pinnacle of adaptive design. Our work confirms that not only is this web type very old, it was also lost in certain lineages of spiders," Jessica Garb of the University of California, Riverside said in a statement.

Garb's team genetically sequenced silk gland tissue from two diverse types of spiders -- one from the net-casting Deinpoidea genus and one from weavers known as Araneoidea.

The sequence suggests the genes used in producing the silk proteins evolved at the same time in both lineages and fits in with fossil evidence suggesting that modern weaving spiders had evolved 136 million years ago or earlier, Garb's team wrote.